Hi,
I have a spare toroidal tranny lying around with 280V secondary Winding, thinking of making something out of it without rewinding anything, I hope to obtain
1) B+ 290~310V @70~90ma
2) B- -60~-80V @ 20~50ma
3) Vbias -15~-25V

After some frantic R&D (Reap and Develope), I came up with the attached circuit. Wonder if it will work safely?

the bias voltage system you have won't work as you think it will, you have no earth reference to anything, so the vbias is essentially floating.

further more if you did connect the right hand side of the choke to earth, the only voltage you would have for the bias suppy would be that dropped across the choke, which in turn is dependent upon the current draw. Not good. it could only be a few volts, thing is, it wouldn't be negative either.

get a copy of duncanamps.com PSUD design tool to design a psu.

If you do need a bias supply, use a pair of caps to connect the ac to a second rectifier, you will be able to create a bias supply from that.

If I were to use another bridge to the tranny via a cap, I'll get another 400V (or -400V depending on how I arrange the diodes), the I'll have to drop to 300V for the rail, that means even more heat generation.

Having an exact winding would be ideal but in this scenario, it would be a great opportunity to learn more about PS and surely we will run into similar situation.

I know of back biasing but all the examples are based on CT windings, I hope we can do it with winding x/o CT.

THe -ve rail generates from the half-wave setup with your high current -80v at the junction of R2, R3 and R4, and your bias voltage controlled by the zener. Set R4 into the 100k plus region to limit current, R3 bleeds the cap C2 down at off time as well as providing a convenient voltage divider. R2 sets the voltage for the -80V tap.

Put the choke in the +ve HT line - I have a feeling it won't enjoy the work put on it by straightening up both full wave and half wave circuits. In any case, you are not going to have a lot of ripple at the -80V tap and the zener will effectively lock the bias tap.

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